This invention relates to processes and apparatuses for the local chemical analysis of solids. There are known processes of this kind wherein the emission of secondary particles from the surface part of the solid under analysis is stimulated by photonic or corpuscular irradiation, and an energy or mass analysis is made of the secondary particles collected, since their characteristics correlate fairly closely with the chemical nature of the emitting atoms of the solid. Amongst the most familiar processes of this kind there may be mentioned the E.S.C.A., Auger spectroscopy, and micro-analysis by x-ray spectroscopy or by secondary ion spectroscopy, known as XPS and SIMS.
In secondary ion mass spectroscopy analysis, as described for instance in U.S. Pat. No. 3,660,655, a primary ion bombardment removes atoms and atom groupings from the surface of a solid target sample. Mass spectrographic analysis of the secondary ions thus removed from the surface has the advantage, since the substance is sampled from the solid by sputtering, of having a high resolution in depth (some tens of Angstroms). Since the ions are produced within a few Angstroms from the surface, such a method can provide a very localized analysis to an accuracy of less than 1 micron by localization of the ion bombardment on a very small elementary area, i.e. by using an ion probe as radiation source. The distribution image of the element or isotope under analysis in a relatively extensive region may also be photographically recorded by filtering the ion image of such region. Unfortunately, only a small proportion of the atoms and atom groupings removed from the solid specimen is ionized and direct analysis of the secondary ions from the surface has two shortcomings. The presence of molecular ions which have either come from the specimen itself or have arisen as the result of chemical reactions between the specimen surface and agents present in the ambient residual atmosphere where the vacuum is incomplete limits the sensitivity at low values. Such molecular ions may lead to ambiguous interpretations if they have the same unit mass as the required ion and if the separating power of the mass spectrometer collecting the secondary emission is less than the possible slight difference in mass between the interfering molecular ion and the required ion.
Quantitative interpretation of results is difficult because the level of ionization -- i.e. the relationship between the number of atomic ions and the number of neutral atoms -- depends for any given element not only on the nature thereof but also upon the nature of the lattice in which such element is present and in particular upon the nature of the chemical bonds between the element and adjacent elements; for instance, the ionization level is much greater (ceteris paribus) in the secondary ionic emission of a compound having an ionic character than in the ionic emission of a metal alloy. The ionizing method disclosed in U.S. Pat. No. 3,660,655 does not completely overcome that problem and has no appreciable effect on the precentage of molecular ions.